You didn't mention 'post' as one of the fields in your question so I'll assume you want to keep it in a separate table.
You can link the two tables ('posts' and the one actually containing the posts [assume 'posttext'] having post stored in field 'postcontent') through the query:
SELECT posts.subject, posts.date, posts.poster, posttext.postcontent
FROM posts, posttext
WHERE posts.id = posttext.id;
this id field could be any string, autonumber or hash generated at the time storing the post.
you could hash the post text for example and store this hash as the id in both the tables ensuring uniqueness.
I would however suggest putting even the postcontent into the 'posts' table thereby saving two efforts:
- storing id in both table would be gone -> that means two queries one for each table now reduced to just one query (both while accessing and writing) so server time saved.
- database size would be mildly reduced because only one index would be used. since there is only one table not two.